Data are limited on the structure of the neonatal colon and its potential function in the electrolyte transport, and thus in the maintenance of fluid and electrolyte balance in neonate. The aim of this project is to define selected aspects of cellular differentiation in the fetal (18-22 days) and neonatal (1-21 days) rat colon. First, the time of appearance and ultrastructural differentiation of the different cell populations will be defined with particular emphasis on the endocrine cells and the vacuolated cells. Then the cytodifferentiation of the apical plasma membrane of the different cell populations will be analyzed using ferritin-conjugated lectins and cationic ferritin as probes. The former will bind to specific cell surface glycoconjugates and differential binding will permit assessment of membrane differentiation. The latter will provide information on the distribution of anionic binding sites as well as the mobility of these sites within the plane of the membrane after incubation at 37 degrees C. Cellular kinetcs including the labelling index and location of the proliferative zone will be determined by autoradiography after incorporation of H3-thymidine into DNA. The migration rate will be analyzed in pulse labelled tissue after time intervals of 12, 24, 48, and 72 hours. Autoradiographs will also be prepared of tissue exposed to a 20 minute pulse of labelled glucosamine, fucose, or N-acetyl-mannosamine for analysis of differential incorporation during glycoprotein synthesis. The rate of intracellular migration of newly synthesized glycoprotein will be analyzed in tissue incubated for an additional 1-12 hours after the pulse label. To determine the potential functional capacity of the neonatal colonic epithelium for sodium absorption changes in Na+/K+ ATPase activity will be measured. This will be assessed by biochemical assays to detect the time of appearance and activity in homogenates, while the in situ activity at the cellular level will be analyzed by a cytochemical reaction on frozen sections which will be measured and quantified with a microspectrophotometer (Vickers, M-85). As with all procedures the crypt to surface differences, regional differences, and changes with increasing age will be noted. This baseline knowledge of the normal sequential changes occurring during cellular differentiation will provide a basis for discrimination of normal processes from abnormal ones in both the neonate and the adult.